got a jca20 head in trade. help me mod it

[dxjxrose]

this is a really cool lil amp but its reallt dead sounding till u hit like 3 or 4 on the master... and id like to get it more into zakk wylde territory any help would be awesome...

[Ostinato Rubato]

If you do a search of "Jet City Mods" in the website search function you will come across a lot of information from some very knowledgeable members who have done various Jet City mods.

[clipless bumper]

Start here: Jet City Lounge

40 page thread on all things Jet City - probably has what you want - but if you can't find it, someone will answer if you ask there.

[Zozobra]

If you're wanting more phase inverter dirt then you can increase the LTPI bias resistor from 470R to anything between 1k to 4k7. This will reduce headroom and encourage more PI distortion so you can run the MV higher for the same volume. If it's just that the amp is muffled/lacks definitions in the high at low MV settings then you can decrease the MV pot value which will let more treble through at the expense of losing a little more signal. It'll be a log 1Meg, try a log 470k. This is a bit of a ballache with the pots being board mounted however.

[dxjxrose]

If you're wanting more phase inverter dirt then you can increase the LTPI bias resistor from 470R to anything between 1k to 4k7. This will reduce headroom and encourage more PI distortion so you can run the MV higher for the same volume. If it's just that the amp is muffled/lacks definitions in the high at low MV settings then you can decrease the MV pot value which will let more treble through at the expense of losing a little more signal. It'll be a log 1Meg, try a log 470k. This is a bit of a ballache with the pots being board mounted however.

thanks... with out a boost and running the gain on boost up to 9 oclock i cant get enough grind outa the amp esp. at low volumes

[MikeO]

The preamp circuit is similar to a JCM 800 in that it uses the same basic design: 3 gain stages with a cathode follower after the 3rd stage to drive the EQ. The values for any given cap or resistor might be different, but those can easily be swapped out for different values if you wanted to make it a 20 watt JCM.

Obviously the 20 watt EL84 based power section is never going to sound like an EL34 power section, but the preamp can be easily modded to sound like the preamp of a JCM 800, or any of the modded variants that are floating around the web.

The muffled sound that you are talking about is a result of the coupling cap between the 1st and 2nd gain stages (C26) allowing too much bass through, which makes the amp sound muffled. I swapped C26 out for a lower value (don't recall what value I changed it to at the moment, but I can dig it up if need be) and it really opened up the preamp.

You can also play with the cathode resistor and plate resistor values to get more grind/gain. The overlooked mod is to change the value of R13. It's a resistor at the end of the preamp that allows some of the signal to bleed off to ground. Stiffening the resistance to ground will keep more of the signal in the circuit and will make the amp sound a little fatter. I have a switch on mine that allows me to switch the resistor out altogether (not allowing any signal to bleed off) and it basically changes the voicing from Tight to Fat. It's great for getting a thicker sound at lower volumes, but I tend to put the switch to the stock resistor value at higher volumes because beyond noon on the master it tends to be overkill. But you might find a happy medium between 220k and no resistor at all if you don't want to drill a hole to install a switch. 500k or 1meg might be the ideal middle ground.

[hardvalve]

Please post clips once you mod it.

[dxjxrose]

well i did the Marshall style depth mod and it made a huge difference... i didnt add a 1uf cap as i didnt get any dc scratchiness in the pot. i add a depth pot to all my amps as i really like to beable to dial in the tightness of the bass.

[dxjxrose]

anyone have the layout for the brown eye mod as i cant find it anywhere

[V2a]

if you want help, I've developed a "mod board" that could probably be dropped into the JCA20. The board is 3"x1" and has two switches that mount to the rear panel. One switch changes the gain structure of the second stage; the other adds in a clipping/SAT circuit.

[Devin]

^ that is really cool!